1. Field of the Invention
This invention relates to an improvement in liquid dispensers that dispense liquid under pressure from an elastomeric bladder.
2. Description of the Prior Art
It is well known that when a fluid is pressured into an elastic container so as to distend the container walls, these elastic walls apply pressure to the liquid. This pressure may be used to propel the liquid out of the container. This relationship finds embodiment in such items as water ballons, pressure accumulators, sprayers, pressure pack barrels and intravenous infusion devices. See for instance the references cited in the file of the above mentioned parent application Ser. No. 344,861. All of these fluid dispensers share the same basic structure. Each has a distensible elastic bag or bladder that applies pressure to an enclosed liquid and forces the liquid through a valve that modulates the rate of flow of liquid from the bladder.
The bladders employed in the prior art are variously described as constructed of natural rubber or of other suitable elastomeric materials. While elastomers generally may be used in such bladders, as a general class they fail to provide a constant pressure on the fluid as the fluid is discharged. Instead, they permit the pressure to decrease significantly as liquid is pumped out. This inability to provide a constant pressure is especially disadvantageous in situations in which a constant flow rate is desired. Only with complicated, continually-changing variable flow valves is it possible to maintain a constant flow as the pressure decreases.